Dies have a tendency to warp or bow during solder reflow processes. This is due to coefficient of thermal expansion (CTE) mismatch between different materials of the die. Due to this warpage, it becomes very difficult to attach the die to a planar substrate in order to achieve a reliable joint. In one example, the die can be a semiconductor chip which requires a very flat surface in order for the solder connections, e.g., C4 connections, to properly connect to a package assembly (planar substrate). In another example, the die can have probe tips assembled on its top surface that are to be used to probe a silicon wafer for electrical tests. These die require they be attached to a rigid ceramic substrate with a very flat probing surface to achieve uniform and reliable contacts with the wafer.
More specifically, a die is made from a silicon wafer which has multiple metallic and/or organic buildup layers on its top surface that act as electrical redistribution layers. The silicon die could also have through-silicon-vias (TSVs) that connect electrical circuitry on these buildup layers to copper interconnects, pillars, etc. The buildup layers, generally, have a much higher thermal expansion than their supporting silicon die. This mismatch causes the composite structure of the die to warp with a rise in temperature. Accordingly the die is no longer flat at the elevated temperatures during reflow.
In the joining process of a typical die to a substrate, the die warp results in non-uniform and unreliable contacts for the thousands of connections between the die and the substrate. In the extreme, the warpage results in non-existent connections (non-wets) between the C4s or copper pillars and their corresponding pads on the substrate, leaving an electrical open circuit. In the example of the probe die which is typically attached to a rigid ceramic substrate, the die warpage gets locked in when the solder solidifies during cool down and makes it difficult to achieve the very tight flatness tolerance needed for the probing process.